While the present invention is applicable to a variety of rotation sensing uses, it will for convenience be described in the context of wheel speed sensing in vehicles, and the described embodiment provides an output signal suitable for use in an anti-lock system on such a vehicle.
Examples of prior wheel speed sensing systems are shown in U.S. Pat. Nos. 3,854,556, 3,938,112, 3,961,215, 3,988,624 and 4,029,180, assigned to the assignee of the present invention. Typically such prior systems provide a ferromagnetic rotor rotatable with a vehicle wheel and a sensing device opposing the rotor across an air gap and fixed against rotation, as to an axle housing of the vehicle. The air gap may be axial or radial. The sensing device typically has been of electromagnetic type with an output signal of frequency proportional to the angular velocity of the rotor. In sensors of this general type, a long continuing problem has been presence of false information or noise in the output signal of the sensing device due to variations in the size of the air gap during operation, in turn due for example to rotor vibration or runout in the direction of the air gap. Such false information or noise in the output signal may cause production of improper lock signals in an anti-lock system.
Past attempts to overcome this problem have included, for example, manufacture of components to close tolerances or elaborate sensor mounting techniques. However, these approaches have been costly and unsatisfactory.
Another prior approach to the problem is disclosed in aforementioned U.S. Pat. No. 3,854,556 in which a pair of pole pieces which are not in simultaneous register with surface interruptions in the rotor are encircled by respective ones of a pair of corresponding coils connected to sum voltages induced by flux change caused by rotor rotation, but to subtract voltages induced by flux changes caused by rotor movement toward and away from the sensing device so that the latter voltages tend to cancel out. However, such prior structure is complicated and made more costly by requiring two separate coils.
Accordingly, the objects and purposes of this invention include provision of:
(1) A rotation sensing system including a wheel speed sensor of the type incorporating a sensing device opposed across an air gap to an annular rotor and wherein the output signal of the sensing device is substantially insensitive to the changes in the width of the air gap during operation.
(2) A system, as aforesaid, in which the sensing device requires only a single coil.
(3) A system, as aforesaid, in which the frequency of the output signal of the sensing device relies not exclusively on a change in amplitude of magnetic flux through the coil, but rather on a change in direction as well as such change in amplitude of magnetic flux through the coil, and wherein such changes in flux direction result from rotation of the rotor with respect to the sensing device but not from changes in the width of the air gap between the rotor and sensing device.
(4) A system, as aforesaid, in which the effect of variations in the width of such air gap during operation is merely to change the amplitude of flux through the coil.
Other objects and purposes of this invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings.